Digital BW, The Print

Clive Moss writes:

> You can't capture the original image in B&W. It is in color.

The image is in color.  You can use a monochrome sensor or B&W film to
capture it in B&W.

> Mapping it to B&W involves a series of implicit decisions
> about what brightnesss to assign assign to a specific colored
> point.

Yup, and they are a function of the _total_ spectrum included in the
original scene.  That spectrum is available at the time of capture, but
it is impossible to reconstruct from any other previous capture.  This
is why you must capture in black and white to begin with.

> For a digital capture, the decision about the relative balance could be
> delegated to the camera manufacturer by switching to a B&W mode, or 
> control could be maintained by the photographer by by playing with the
> RGB balance in channel mixer.

No, this isn't possible.  In the case of a digital capture, the capture
device is fixed in its capabilities.  The total spectrum of the original
scene is converted to three color values in a fixed and finite way.
Thereafter, it _cannot_ be transformed into the equivalent of an
entirely different capture.

Using a black and white mode doesn't help, because the sensor itself
still is covered with a matrix color filter.  You must remove the color
filter in order to get true black and white, in which case the
characteristics of the capture will be determined by the spectral
sensitivity of the chip (quite wide and flat for CCDs, as long as there
is an infrared filter in place).